The present invention relates generally to high strength, low weight structures comprising an amorphous core or filler material surrounded by and fused to a shell composed of resin-coated high strength composite filaments or fibers. More specifically, the present invention relates to a lightweight, high strength and stiff tennis racket having improved energy absorption characteristics, structural endurance and playability wherein the amorphous core is made from a combination of flexible polymer foam and lightweight honeycomb reinforcement layers or structures.
There is presently available to today's tennis player, a wide variety of tennis rackets. These tennis rackets are made from various different materials and are available in numerous different sizes, weights, and configurations.
One of the more popular tennis frames developed recently is based on the use of resin-coated composite filamentary materials such as graphite fiber, glass fiber, boron filaments, kevlar filaments or any combinations thereof. By far, the most popular composite racket is made from resin impregnated graphite fibers.
Graphite fiber composite materials were originally developed for aerospace applications in high performance aircraft and missile structures. Due to the inherent lightweight and high strength of graphite composites, they were also found especially well-suited for use in sports equipment. As a result, graphite composites have been widely used in the production of lightweight, high strength and high performance tennis racket frames. When cured at elevated temperatures, the resin-coated graphite fibers form a high strength, rigid and lightweight structure which is particularly well-suited for providing a high performance tennis racket.
Basically, tennis frames made from graphite composites include a tubular rigid composite frame structure or shell. The hollow core present in such a tubular frame structure is typically filled with a variety of core materials. The properties and configurations of both the rigid composite shell and core structure are important in providing a tennis racket with desired performance qualities. Graphite fibers of various different sizes impregnated with a wide variety of resin materials are commercially available. Many of these commercially available graphite composite materials have been used to provide entirely adequate rigid tubular tennis frame structures. With regard to the core structure, however, there has yet to be developed a core material or structure which has been found entirely adequate for use in the high performance graphite tennis rackets.
An optimum core structure should provide good energy absorption characteristics to reduce shock and vibration which otherwise would be present in a hollow tubular frame structure during ball impact. Further, the mass distribution of the core material throughout the tennis frame tubular structure should be easily varied. This variable mass distribution allows fine tuning or balancing of mass between the frame head and handle to enhance desired performance characteristics. The core structure should also be resistant to degradation and decomposition due to shock and vibration over long periods of racket use. In addition, it is desirable that the core material fuse or otherwise bond to the interior of the rigid composite shell to insure a solid vibration-free feel during racket use.
Another important property desirable in an optimum core material is the ability of the core material to expand or otherwise provide internal pressurization during molding of the graphite racket. Typical production of graphite composite/core tennis rackets involves surrounding the core with graphite fibers in specific orientations. The graphite fiber/core structure is then molded at elevated temperatures to provide the desired structural shape. The internal pressure for insuring that the rigid graphite shell is molded properly is typically and most conveniently provided by the core itself. The core material's ability to expand or otherwise generate internal pressure is therefore an important quality which is desirable in commercial process for producing such graphite composite tennis rackets.
Foamable or intumescent resinous compositions have been utilized as suitable core materials. Foamable materials are desirable since they provide the necessary internal pressure during the molding operation. Typically, the resin compositions are mixed with various additives such as barium sulfate, chopped cork, glass, asbestos, fibers, mica flakes and the like. These additives are used for various reasons ranging from control of density within the core to low density fillers to produce a lighter weight racket. These resinous core compositions typically include epoxy of phenolic resins. The core characteristics range from stiff, hard and brittle compositions, to those compositions having consistencies of firm putty or molding clay. Although many of the core materials presently being used in graphite frames have been found adequate for their intended purpose, problems have been experienced with premature deterioration and crumbling of the core material resulting in loss thereof through stringing holes or other openings in the racket frame.
In response to the need for an optimum core material and structure, one of the coinventors of the present application developed a flexible core structure made from plasticized polyvinyl chloride which was found to provide enhanced racket performance characteristics. The elastic core and the composite structure based thereon is the subject of a copending patent application entitled "ELASTIC CORE COMPOSITE STRUCTURE" filed July 31, 1981, and given Ser. No. 288,999. This new elastic or flexible polymer core is based on a flexible vinyl foam which is most preferably made from a plasticized polyvinyl chloride in which a suitable blowing agent is dispersed.
Although this new flexible foam core structure provides a tennis racket frame with desirable high performance characteristics, a core structure made entirely from flexible foam may not provide the desired high strength and stiffness required for certain performance applications. Accordingly, it would be desirable to provide a new core structure which not only includes the advantages of the previously described elastic or flexible cores, but also includes enhanced structure reinforcement to provide an especially strong reinforced composite tennis frame structure.